1. Field of the Invention
The present invention relates to a mobile communication and, more particularly, to a method for generating and transmitting an optimal cell ID code for site selection diversity transmission (SSDT) in a third generation mobile communication.
2. Description of the Background Art
In general, a code division multiple access (CDMA) system is well known to be interference-limited and require power control to solve the near-far problem and slow shadow fading.
In addition, a third generation mobile communication system based on the CDMA, such as a universal mobile telecommunications system (UMTS), employs fast power control so as to increase system capacity by compensating radio channel variations caused due to multi-path fading by the users. In the fast power control, a user equipment (UE) needs to measure a signal-to-interference ratio (SIR), compare it with a target SIR, and transfers a transmit power control (TPC) command comprising one or more bits to a base station (cell). Upon receiving the TPC command, the base station controls transmission power by the unit of a fixed amount according to the TPC command.
Unfortunately, the fast power control brings about a problem in controlling downlink power control when the UE simultaneously communicates with multiple base stations related to a soft handover.
In the soft handover, all the base stations related to the soft handover simultaneously transmit downlink signals to the UE and independently follows the TPC command received from the UE. Therefore, if there is an error in the uplink TPC command, the transmission power of the base stations would drift.
An increase in the difference between the transmission powers of different base stations causes a reduction in soft handover diversity gain and an increase in the interference to other users.
To overcome this problem, various power balancing schemes have been proposed to compensate for the drift of transmission power. Of the schemes, a site selection diversity transmission (SSDT) has been adopted as a power control scheme in the soft handover environment by the 3rd-generation partnership project (3GPP).
In SSDT, data are transmitted from only one of the cells involved in the soft handover, i.e., the cell with the least instantaneous path loss to the target UE.
The UE selects one of the cells of its active set as ‘primary’, and other remaining cells are classified as ‘non-primary.’
A primary objective of the SSDT is to transmit on the downlink from the primary cell, thus reducing the interference caused by multiple transmission in a soft handover mode. A second objective is to achieve fast site selection without network intervention, thus maintaining the advantage of the soft handover.
In order to select a primary cell, each cell is assigned a temporary identification (ID) and UE periodically informs linked cells of a primary cell ID. The non-primary cells selected by UE switch off the transmission power. The primary cell ID is delivered by UE to the cells included in the active set via uplink FBI field. SSDT activation, SSDT termination, and ID assignment are all carried out by higher layer signaling.
In SSDT, in order to avoid the channel disconnection due to failure of primary cell selection resulting from a bad channel quality, conditions for being a non-primary cell is quite strict.
The UE periodically transfers the primary cell ID code through a portion of an uplink feedback information (FBI) field assigned for SSDT use (FBI S field). A cell recognizes its state as non-primary if all the following conditions are satisfied:                (1) A received ID code is not identical to its own ID code.        (2) A value of a received uplink signal quality is greater than a predetermined value defined by the network.        (3) In case of an uplink compressed mode, a bit loss of the ID code is smaller than NID/3 (as a result of uplink compressed mode), wherein NID is the number of bits in the ID code (after puncturing if puncturing is performed). Otherwise the cell recognizes its state as primary.        
In SSDT, a dedicated physical data channel (DPDCH) is transmitted only by the primary cell, whereas a dedicated physical control channel (DPCCH) is transmitted by the non-primary cells as well as the primary cell because the DPCCH includes control information.
FIG. 1 shows a structure of the uplink DPDCH and the uplink DPCCH. Each radio frame with a length of 10 ms is divided into 15 slots, each of length Tslot=2560 chips corresponding to one power-control period. The DPDCH and DPCCH are always frame-aligned with each other. As shown in FIG. 1, the DPCCH includes pilot bits to support channel estimation for intervention detection, transmit power control (TPC) commands, feedback information (FBI), and an optional transport format combination indicator (TFCI).
The FBI bits are used to support techniques requiring feedback from the UE to the UTRAN access point such as a closed loop mode transmit diversity and SSDT. The structure of the FBI field is shown in FIG. 2.
As shown in FIG. 2, the FBI field includes an S field for SSDT signaling and a D field for closed loop mode transmit diversity signaling. The S field consists of 0, 1, or 2 bits and the D field consists of 0 or 1 bit. The maximum size of FBI field NFBI is 2 bits. If the FBI field is not entirely filled with S field or D field, the FBI field would be filled with “1”. When the NFBI is 2 bits, S field is 0 bit, and D field is 1 bit, left side field is filled with “1” and right side field is D field.
Each cell is given a temporary ID during SSDT and the ID is used as site selection signal, The ID is given as a binary bit sequence. There are three different lengths of coded ID available denoted as “long ”, “medium ”, and “short. The network decides which length of coded ID is to be used. Setting of ID codes for 1-bit and 2-bit FBI is shown in below table 1 and table 2, respectively.
TABLE 1Settings of ID codes for 1 bit FBIID codeID label“long”“medium”“short”A000000000000000(0)000000000000B101010101010101(0)101010101001C011001100110011(0)011001111011D110011001100110(0)110011010010E000111100001111(0)000111100111F101101001011010(0)101101001110G011110000111100(0)011110011100H110100101101001(0)110100110101
TABLE 2Settings of ID codes for 2 bit FBIID code(Column and Row denote slot positionand FBI-bit position.)ID label“long”“medium”“short”A(0)0000000(0)000000(0)0000000(0)000000B(0)0000000(0)000000(1)1111111(1)111111C(0)1010101(0)101101(0)1010101(0)101101D(0)1010101(0)101101(1)0101010(0)011010E(0)0110011(0)011011(0)0110011(0)011011F(0)0110011(0)011011(1)1001100(0)100100G(0)1100110(0)110110(0)1100110(0)110110H(0)1100110(0)110110(1)0011001(1)001001
The ID code bits shown in table 1 and table 2 are transmitted from left to right. In table 2, the first row refers to the first FBI bit in each slot, and the second row refers to the second FBI bit in each slot.
The ID codes are transmitted after being aligned to the radio frame structure (that is, the ID codes shall be terminated within a radio frame). If the FBI is short of a space for transmitting the last ID code within a frame, the first bit(s) of the ID code are punctured. The bit(s) to be punctured are shown in brackets in table 1 and table 2.
As shown in table 1, the minimum Hamming distance of the ID codes for 1 bit FBI is 8 for long code of 15 bits (dmin=8), 4 for medium code of 8 bits (dmin=4), 4 for punctured medium code of 7 bits (dmin=4), and 2 for short code of 5 bits (dmin=2).
Meanwhile, referring to table 2, the minimum Hamming distance of the ID codes for 2 bit FBI is 8 for long code of 16 bits (dmin=8), 7 for punctured long code of 14 bits (dmin=7), 4 for medium code of 8 bits (dmin=4), 3 for punctured medium code of 6 bits (dmin=3), and 3 for short code of 6 bits (dmin=3).
In view of Hamming distance, it is noted that the minimum Hamming distance of the ID code for 1 bit FBI is not changed even after the bit puncturing is performed, However, in case of the ID codes for 2 bit FBI, notably, the minimum Hamming distance is reduced in the long code and minimum code because of the bit puncturing.
FIG. 3 shows how the long ID code for 2-bit-FBI is allocated in the radio frame. One radio frame consists of 15 slots and each slot is given 2 bits for FBI, so that the total number of FBI bits per frame is 30. Accordingly, a long ID code of 16 bits is carried by the first 8 slots of the frame and a long ID code of 14 bits obtained by puncturing first two bits of the 16 bit-long ID code, is carried by the remaining 7 slots of the frame, thereby completing transmission of the ID codes in one radio frame.
However, such cell ID code generation and transmission method the following problem. That is, in the case using the long code for the 2 bit FBI, the minimum Hamming distance of the ID codes is reduced due to the bit puncturing, resulting in degradation of an error correction efficiency of the cell ID code.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.